Japanese Patent Application Publication No. 2010-109117 discloses a switching device provided with a GaN layer and an AlGaN layer. The GaN layer forms a heterojunction with the AlGaN layer. Due to this, a two-dimensional electron gas (hereafter referred to as “2DEG”) is generated at an interface between the GaN layer and the AlGaN layer. A gate electrode is provided on the AlGaN layer. When a gate potential is lowered than a threshold, the 2DEG under the gate electrode vanishes. At this occasion, the 2DEG in the heterojunction separates into a drain electrode side and a source electrode side. Due to this, no current flows between the drain electrode and the source electrode. That is, the switching device turns off. When the gate potential is increased than the threshold, the 2DEG reappears under the gate electrode, and the drain electrode and the source electrode are connected by the 2DEG. Thus, current flows between the drain electrode and the source electrode. That is, the switching device turns on. Accordingly, the switching device can be switched on and off by controlling the gate potential. Notably, the aforementioned threshold may be higher than 0V (that is, 0V means a same potential as a source electrode) or may be lower than 0V. A switching device with the threshold higher than 0V is of a normally-off type, and a switching device with the threshold lower than 0V is of a normally-on type. Further, there also is a switching device in which a two-dimensional hole gas (hereafter referred to as “2DHG”) is generated at a heterojunction. The switching device that utilizes the two-dimensional hole gas turns on when its gate potential is lowered than the threshold, and turns off when the gate potential is increased than the threshold
In any of the aforementioned switching devices, an electric field is generated in a semiconductor layer between the gate electrode and the drain electrode when the switching device is turned off. At this occasion, in general, the semiconductor layer in a vicinity of an end of the gate electrode on the drain electrode side (for example, the AlGaN layer in the case of Japanese Patent Application Publication No. 2010-109117) is prone to electric field concentration. If a high electric field is generated in the semiconductor layer, a voltage resistance of the switching device is deteriorated. With respect to this, the switching device of Japanese Patent Application Publication No. 2010-109117 includes a field plate extending from the gate electrode toward the drain electrode side above the AlGaN layer. The field plate is arranged on an insulating layer covering a front surface of the AlGaN layer. Since the field plate is connected to the gate electrode, it has a substantially same potential as the gate electrode. When the field plate is arranged as aforementioned, the electric field concentration in the vicinity of the end of the gate electrode on the drain electrode side is mitigated.